It is common in the textile material processing industry to provide sensors which can detect irregularities in the textile feedstock during processing. These sensors are combined with an interface to the drive motor power supply or clutch to create a "stop motion" which stops the processing operation in response to the detected irregularity. For example, the high speed textile drawing process takes a feed of multiple strands of slivers composed of textile fibers and produces a single drawn sliver. It is important for the quality of the drawn sliver that the input feed slivers do not part during the drawing operation, or if so, that the drawing operation be immediately stopped until the parted sliver can be pieced together. Although the sliver may be quickly and easily pieced together by hand, a continuous monitoring system is desirable to detect sliver parting and to concurrently stop the drawing process.
A powered creel is often used in conjunction with the high speed draw frame to provide an uninterrupted supply of the sliver stock to the draw frame and to eliminate or reduce stretching of the individual slivers being fed. The powered creel uses a series of driven rollers to move slivers from a series of storage cans to the drafting rolls of the draw frame. Although the powered creel reduces the possibility of sliver parting, quality control considerations still require that the sliver stock fed into the drafting zone of a high speed draw frame be continuously monitored to detect any parting of the sliver which would affect the drawn fiber weight and also the blend if different types of sliver are being processed.
A conventional stop motion for detecting the parting of sliver may typically be a set of two electrically conductive rollers through which the sliver passes. When the sliver parts, leaving no sliver to separate the rollers, an electrical circuit is completed and the drawing process is stopped. One of the disadvantages of this type of detector is that fiber lint and residue from the slivers have a tendency to accumulate upon the moving rollers and prevent the completion of the electrical connection. The drawing process continues despite the absence of one of the feed slivers, thereby producing a lightweight sliver. Other alternative detectors, including switches and photoelectric sensors, may also be rendered inoperable by the accumulation of fiber lint and residue material.
A further disadvantage of some conventional stop motions for sliver, is that they must necessarily contact the sliver and impose a drag upon the sliver to sense a parting. Unfortunately, this drag results in the unwanted effect of increasing the likelihood and frequency of sliver parting.
Mechanical switch stop motions which rely on electrical contact between mechanical components, may be unable to detect a parting of the sliver in the portion of the sliver path between the stop motion and the draw frame. A parting in this portion of the sliver path will cause the sliver in the switch to stop moving; however, electrical contact is not established to trigger the stop motion because the sliver still separates the two mechanical components of the switch. A mechanical switch may only be effective where the parting occurs between the sliver storage cans and the stop motion.
In a related textile field of yarn creeling, capacitive sensing stop motion devices have been developed to detect when a yarn end is down. Textile yarns carry an electrostatic charge, caused by unwinding the yarn from the supply package in the creel and by friction with yarn guides. Similarly, U.S. Pat. No. 4,914,785 to Hauner discloses a single sliver guide with a capacitive sensor held therein by a casting compound with each single sensor located immediately proximate a single sliver storage can.